<?php

require_once('../wp-blog-header.php');

$wpdb->query (
    $wpdb->prepare (
        "UPDATE {$wpdb->posts} SET post_modified = %s, post_modified_gmt = %s
        WHERE ID IN (SELECT post_id FROM {$wpdb->postmeta} WHERE meta_key =  %s)",
        current_time ('mysql'), current_time ('mysql', 1), 'YOUR_META_KEY'
    )
);

?>
